It is known from the prior art to colour match a variety of objects simply by eye and to compare the colour with a reference colour guide. The colour guide is usually held in proximity to the test object for a direct comparison. This is a purely subjective evaluation and can lead to a colour mismatch since one person's assessment may differ from another's. Moreover, the perception of colour is dependent on a number of factors such as the background lighting conditions and texture of the object itself. An object with a pitted surface may, if viewed in one direction, cast shadows on its own surface thus distorting the overall colour of the object. Alternatively, a smooth shiny object can reflect light from its surface leading to bright spots similar colour distortions.
In the restoration or replacement of a tooth or set of teeth, it is important accurately to select the correct tooth colour so as to match, not only the adjacent tooth in colour and shape, but to match the entire set of teeth in overall colour harmony and surface contour profile. However, it is difficult to critically assess highly reflective surfaces such as the enamel of teeth. It is known from the prior art to quantify the colour of teeth by manually comparing a patient's natural teeth with a set of “shade guides”. These guides typically comprise a row of substantially flat, plastic tooth shaped items mounted on a board in ascending grades of shades. The first step in the colour determination process is made subjectively by the dentist or dental technician, by holding the shade guide next to the patient's own natural teeth and attempting to find the best match. This can be problematic because tooth colour is affected not only by ambient light colour/intensity in the surgery i.e. fluorescent or natural light, but also by the surrounding colour of the patient's own clothing or make-up/complexion. In addition this step is dependent on the visual acuity and experience of the dentist or dental technician.
Once the dentist or dental technician has made his/her choice of best match colour from the shade guide the next step in the process is to relay the information to a dental laboratory technician who then constructs the dental prosthesis, typically from a set of pre-coloured components. The information he/she may receive is that the dental prosthesis required is a mix between two or more of the shades on the guide. In this step of the overall process, there is a dependency on a yet further subjective colour assessment by the dental laboratory technician when mixing appropriate ratios of the pre-coloured components to the specified recipe. Once constructed, the finished product is then returned to the dentist for fitting into a patient's mouth. It is only after the dental prosthesis has been constructed that it becomes apparent if the colour match was accurately evaluated by the dentist or dental technician and subsequently constructed by the dental laboratory technician. It will be appreciated this process often results in unacceptable levels of colour mismatching so that a second or replacement dental prosthesis needs to be constructed at a substantial cost and inconvenience to the patient, dental professional and dental prosthesis manufacturer.
Methods which have been attempted to try to minimise human error when assessing tooth colour include:                illuminating the patient's mouth in a controlled manner and comparing the natural teeth to a reference shade guide set illuminated under similar conditions. The problem with this method is that it does not completely eliminate variations in ambient lighting conditions.        photographing the patient's mouth with a reference shade guide in the frame. The problem with this method is that the equipment can be bulky and that the colour in the photograph may be distorted and/or misrepresented through the process of developing and producing the photograph. In addition, the flash from the camera causes high levels of reflection from the tooth surface.        videoing the patient's mouth with a reference shade guide in the frame. The problem with this method is that the equipment can be bulky and that the colour in the video may not be accurate.        manually drawing and painting/colouring an artist's impression of a tooth. The problem with this is that it can be expensive, time consuming, and it is not independent of ambient lighting conditions.        
None of the prior art methods is capable of capturing an exact colour image of a natural tooth. This is partly because of the inherent properties of teeth themselves. Natural teeth are curved, not uniformly smooth and the colour distribution of the tooth is not even or uniformly distributed throughout the tooth. Natural teeth are translucent on their surface, the transparencies of dentine and enamel are difficult to correct for when representing the colour of a tooth. Teeth are light reflective which results in bright spots and bright lines. All of these factors contribute to the difficulty in accurately capturing a colour image of a tooth.
A problem not addressed by any of the prior art methods is the subjective colour assessment which the dental laboratory technician has to make when given a recipe or image to work to, so as to construct the dental prosthesis. Some methods have made improvements in the standardisation of assessing the colour of the patient's tooth in the first instance, but problems remain with human errors in constructing and colour matching the prosthesis to a recipe.
A method of accurate colour image capture and construction of a prosthesis to that colour would offer immediate advantage over the prior art.